全文获取类型
收费全文 | 5108篇 |
免费 | 252篇 |
国内免费 | 49篇 |
专业分类
化学 | 3745篇 |
晶体学 | 45篇 |
力学 | 112篇 |
综合类 | 1篇 |
数学 | 456篇 |
物理学 | 1050篇 |
出版年
2023年 | 25篇 |
2022年 | 53篇 |
2021年 | 92篇 |
2020年 | 81篇 |
2019年 | 107篇 |
2018年 | 81篇 |
2017年 | 67篇 |
2016年 | 171篇 |
2015年 | 153篇 |
2014年 | 189篇 |
2013年 | 306篇 |
2012年 | 341篇 |
2011年 | 386篇 |
2010年 | 256篇 |
2009年 | 225篇 |
2008年 | 316篇 |
2007年 | 270篇 |
2006年 | 272篇 |
2005年 | 276篇 |
2004年 | 218篇 |
2003年 | 207篇 |
2002年 | 184篇 |
2001年 | 117篇 |
2000年 | 108篇 |
1999年 | 56篇 |
1998年 | 43篇 |
1997年 | 53篇 |
1996年 | 68篇 |
1995年 | 48篇 |
1994年 | 44篇 |
1993年 | 62篇 |
1992年 | 48篇 |
1991年 | 40篇 |
1990年 | 40篇 |
1989年 | 48篇 |
1988年 | 28篇 |
1987年 | 26篇 |
1986年 | 30篇 |
1985年 | 29篇 |
1984年 | 20篇 |
1983年 | 16篇 |
1982年 | 17篇 |
1981年 | 26篇 |
1980年 | 15篇 |
1979年 | 18篇 |
1977年 | 19篇 |
1976年 | 19篇 |
1975年 | 11篇 |
1974年 | 19篇 |
1973年 | 11篇 |
排序方式: 共有5409条查询结果,搜索用时 0 毫秒
81.
82.
[reaction: see text] A zirconocene-mediated ring contraction of 4-vinylfuranosides generated either from d-arabinose or d-glucose is followed by sequential oxidation to the ketone and alkynyl Grignard addition. The resulting cis-cyclobutanediols are subjected in turn to thermal rearrangement and intramolecular oxymercuration-demercuration. The regiochemistry of the final ring closure is controlled by the nature of R. 相似文献
83.
A solution of carprofen in methanol at a concentration of 2.74 x 10(-2) mg/mL is subjected to photoirradiation using a Hanovia 200-W high-pressure Hg lamp for 9 h. In total, seven photodegradation products are separated, and their quasimolecular ions are subsequently determined online using a liquid chromatography (LC)-electrospray ionization (ESI)-mass spectrometry (MS) method. The high-performance LC consists of an Inertsil 5 ODS-80A (2.1- x 150-mm) column. The mobile phase is initially CH(3)CN. NH(4)OAc (20mM in de-ionized H(2)O) is 43:57 (v/v), and after 14 min it is CH(3)CN. NH(4)OAc (20mM in de-ionized H2O) is 54: 46 (v/v). The UV detector was set at 260 nm. The parameters of LC-MS for mass determination involves an atmospheric pressure ionization electron spray interface with a negative mode of polarity (ESI(-)). The chemical structures of the degradants are elucidated based on the mass-to-charge ratio of the quasimolecular ions and the molecular weight changes by comparison with the parent drug (carprofen). The degradation proceeds via an initial dechlorination. A dechlorination or esterification reaction is competed with decarboxylation. This finding is in accordance with our previously reported result of first order photodecomposition kinetics for carprofen. 相似文献
84.
A new calix[4]crown chemosensor based on dual sensing probes reveals Pb2+ ion selectivity over other metal ions, which arises from a hypsochromic shift of azo units in UV spectrum as well as a fluorescence enhancement of pyrenyl parts in fluorescence spectrum via a suppressed FRET. 相似文献
85.
The room-temperature phosphorescence (r.t.p.) and fluorescence spectra of benzoquinoline isomers are investigated. The isomers can be resolved into the linear or angular subgroups on the basis of their fluorescence and r.t.p. spectra by using conventional fixed excitation. Second-derivative and synchronous scanning techniques can be combined to improve the selectivity of the r.t.p. and fluorescence methods. These simple luminescence techniques were used to estimate three benzoquinoline isomers in a coal tar fraction. Direct analysis of this complex sample allowed acridine to be estimated and upper limits to be provided for benzo(h)quinoline and phenanthridine; the presence of three other isomers was not detected. Comparative studies with data obtained by high-performance liquid chromatography are reported. 相似文献
86.
B. D. Silverman J. W. Bartha J. G. Clabes P. S. Ho A. R. Rossi 《Journal of polymer science. Part A, Polymer chemistry》1986,24(12):3325-3333
The calculated carbon 1s (C1s) core energy-level positions of PMDA-ODA polymide and of its polyamic acid precursor are compared with the level positions inferred from XPS measurements. For the polyamic acid, calculation and experiment both yield a difference of approximately 1 eV between the carboxylic acid and the amide carbonyl C1s level positions. The difference in shape between the main C1s XPS peaks of the polyamic acid and polyimide is shown to be related to the difference in C1s core energy-level shifts of the carbon atoms composing the benzene ring adjacent to the amide or imide group. The planar imide or PMDA structure apparently yields larger core level shifts for these atoms. We have previously designated these atoms as “imide carbon atoms” (C-Im) to distinguish them from the aromatic carbon atoms (C-C) of the ODA part of the polymeric repeat unit. Comparison of the carbonyl XPS band intensities with the main peak intensities for the polyamic acid, as well as for the polyimide, suggests that there is a carbonyl deficiency at the surface of both of these materials. 相似文献
87.
88.
Yoshihisa Kurasawa Tomoyoshi Hosaka Kazue Ikeda Yuko Matsumoto Aiko Ishikura Atsushi Takada Ho Sik Kim Yoshihisa Okamoto 《Journal of heterocyclic chemistry》1994,31(2):527-533
The 3-(arylhydrazono)methyl-2-oxo-1,2-dihydroquinoxalines 9–11 were synthesized by the reaction of the quinoxalines 6–8 with various p-substituted benzenediazonium salts. Compounds 9–11 showed the tautomeric equilibria between the hydrazone imine A and diazenyl enamine B forms in dimethyl sulfoxide media. The substituent effect on the tautomer ratios of A to B was studied by the nmr spectroscopy to clarify that the presence of the ester group R2 on the hydrazone carbon and electron-donating p-substituent R1 on the side chain benzene ring exhibited a tendency to increase the ratios of the tautomer A . 相似文献
89.
Jang MJ Lee MJ Park HY Bae YS Min DS Ryu SH Kwak JY 《Experimental & molecular medicine》2004,36(2):172-178
Agents that elevate cellular cAMP are known to inhibit the activation of phospholipase D (PLD). We investigated whether PLD can be phosphorylated by cAMP-dependent protein kinase (PKA) and PKA-mediated phosphorylation affects the interaction between PLD and RhoA, a membrane regulator of PLD. PLD1, but not PLD2 was found to be phosphorylated in vivo by the treatment of dibutyryl cAMP (dbcAMP) and in vitro by PKA. PKA inhibitor (KT5720) abolished the dbcAMP-induced phosphorylation of PLD1, but dibutyryl cGMP (dbcGMP) failed to phosphorylate PLD1. The association between PLD1 and Val14RhoA in an immunoprecipitation assay was abolished by both dbcAMP and dbcGMP. Moreover, RhoA but not PLD1 was dissociated from the membrane to the cytosolic fraction in dbcAMP-treated cells. These results suggest that both PLD1 and RhoA are phosphorylated by PKA and the interaction between PLD1 and RhoA is inhibited by the phosphorylation of RhoA rather than by the phosphorylation of PLD1. 相似文献
90.
Tagless extraction-retentate chromatography: a new global protein digestion strategy for monitoring differential protein expression 总被引:5,自引:0,他引:5
A new global protein digestion and selective peptide extraction strategy for the purpose of monitoring differential protein expression, coined as tagless extraction-retentate chromatography, is introduced. Target protein populations are firstly digested under reduced and alkylated conditions, and resultant peptides selectively extracted via covalent attachment to methionine residues by bromoacetyl reactive groups tethered to the surface of glass beads packed in small reaction vessels. After conjugation, reactive beads are stringently washed to remove nonspecifically bound peptides and then later treated with beta-mercaptoethanol to release captured methionine peptides in their nascent state, without complicating affinity tags. Recovered methionine containing peptides are profiled using the surface-enhanced laser desorption/ionization (SELDI) retentate chromatography mass spectrometry (RCMS) method. Selected peptides are further studied employing ProteinChip tandem mass spectrometry (MS/MS) analysis to identify their parent proteins. This approach has been applied to an Escherichia coli lysate model system and has demonstrated facility in reducing global digest complexity, sensitivity to low protein expression levels, and significant quantitative capability. It is envisioned that tagless extraction-RCMS will evolve to be a valuable approach for both basic research and clinical proteomics endeavors. 相似文献